High speed analog-to-digital converters used to digitize sensor signals, notably in the field of radar or telecommunications, have an input stage comprising a sample-and-hold device allowing the input signal to be frozen during the quantification phase. The performance of the converters therefore depends heavily on the performance of the sample-and-hold device.
The sample-and-hold process consists in charging a capacitor, referred to as a holding capacitor, during a first phase of tracking or sampling the input signal, and in isolating this capacitor from the input signal which may continue to vary, thus blocking the value of the voltage on the capacitor terminals. The quantification of the signal can be carried out during a second hold phase.
A problem which arises lies in implementing this switching in such a way that the sampled signal reproduces the input signal as faithfully as possible at the blocking time.
Most known solutions use the switching of a diode or of a base-emitter junction. The diode is conducting in the track phase and is blocked by controlling the voltage on its anode during the hold phase.
A first solution is the diode bridge, notably described in J. R. Gray and S. C. Kitsopoulos “A Precision Sample-and-Hold Circuit with Subnanosecond Switching” IEEE Transactions on Circuit Theory, CT11, September 1964, pages 389-396, with multiple variants.
A switched-tracker solution, requiring only a current source, is preferred in integrated circuits. It is notably described in document U.S. Pat. No. 3,643,110, multiple variants being used.
These solutions have a number of disadvantages. In particular, a diode switching device is strongly non-linear.
During the reversal, an injection of charges into the holding capacitor occurs, introducing a parasitic voltage which is added to the signal, this voltage also being referred to as a “pedestral error”.
Furthermore, the isolation between the input and the output is weak since the signal passes across the junction capacitor of the diode.
Finally, this type of switching device is necessarily:
preceded by an input stage in order to isolate the input, thus introducing an additional power consumption and non-linear effects;
followed by an output stage in order to isolate the capacitor, introducing an additional power consumption and introducing non-linear effects.